Make EndpointId nullable and add EndpointUri property

The `EndpointId` property in the `CreateRecActionCommand` class
was updated from a non-nullable `long` to a nullable `long?`
to allow it to hold `null` values.

Additionally, a new nullable `string?` property `EndpointUri`
was introduced to support specifying an optional endpoint URI.

src/ReC.Application/RecActions/Commands/CreateRecActionCommand.cs

@@ -6,7 +6,9 @@ public record CreateRecActionCommand
 
     public bool Active { get; init; } = true;
 
-    public long EndpointId { get; init; }
+    public long? EndpointId { get; init; }
+
+    public string? EndpointUri { get; init; }
 
     public string Type { get; init; } = "GET";